Chemical and immunologic studies are in progress involving the cell walls of Histoplasma capsulatum and Blastomyces dermatitidis. Both phases of these two fungi are being included but the primary emphasis is on the yeast phase. On the basis of chitin and monosaccharide analyses, as well as ethylenediamine extraction, we described two chemotypes of H. capsulatum yeast phase cell walls, one of which was similar to B. dermatitidis. These three groups, i.e., H. capsulatum chemotype I and II and B. dermatitidis, are now being examined for possible differences in the types of glycosidic bonds present in the wall, and for the presence of unique or similar proteinaceous components. The immunologic studies involve both humoral and cell-mediated responses elicited in guinea pigs by immunization with cell walls or fractions thereof. Complement fixation, the fluorescent antibody test and immunodiffusion are being used to evaluate the sera generated in the immunizations, while skin testing and migration inhibitory factor assays are being employed for the cell-mediated response.